A particular advantage of plasma-assisted etching of materials over wet etching processes is the directionality of the plasma-assisted etching due to energetic ion bombardment of the substrate. Plasma etching is indispensable in semiconductor manufacturing for reducing device size and increasing the aspect ratio of etched features. The energy provided to the substrate surface from ion impact can enhance chemical reactions via several mechanisms, demonstrated both in simulations and ion beam experiments, with significant advantages in controlling the profile of etched features, in etch selectivity, and in film quality in plasma enhanced chemical vapor deposition processes (PECVD). In typical plasma processes used in semiconductor manufacturing, the ion energy is coarsely controlled by varying the amplitude of a radio frequency (RF) sinusoidal bias voltage that is applied to the substrate electrode. However, the resulting energy distribution function (IEDF) is generally broad, which limits the ability of the plasma process to further improve such characteristic as etch feature profiles, etch selectivity and PECVD film quality.
The use of the conventional sinusoidal substrate bias for IEDF control is limited by physical constraints. The ion mean-free path in the sheath region, the ion sheath transit time, and the substrate bias voltage waveform, which determines the sheath voltage drop between the plasma and the substrate, are the primary factors that determine the IEDF at the substrate. In the high pressure and/or high substrate voltage conditions typically used in reactive ion etching (RIE), the mean-free path of ions is comparable to or even shorter than the sheath thickness. In such cases, the IEDF at the substrate is broadened due to the collision in the sheath region regardless of the bias voltage waveform. In contrast, high density plasmas (HDP) used for semiconductor processing, characterized by high plasma density, low pressure and lower average substrate voltage, typically have collisionless sheaths at the substrate. The typical IEDF at the substrate for a HDP process is a bimodal curve which coalesces into a single peak when the substrate bias frequency is sufficiently high as compared to the ion plasma frequency. The variation in ion energy arises from the temporal modulation of the sheath voltage. If the ion transit time across the sheath is short compared to the RF period, the bombarding energy of any given ion will correspond to the sheath voltage at the moment it reaches the sheath edge. For ion transit time long compared to the RF period, the ion energy more closely corresponds to the average sheath voltage. Although increasing the bias frequency is one route that has been considered as a method for narrowing the IEDF, it suffers from two fatal limitations. First, the width of the IEDF is ion mass dependent, and, even for low bias frequencies, tends to be wide for low mass ions that are often produced in processing plasmas. Second, at sufficiently high frequencies, the RF wavelength becomes comparable to the substrate dimensions, and bias voltage non-uniformities across the substrate surface develop, leading to unacceptable process non-uniformities.
As discussed above, the IEDF for the conventional RF sinusoidal bias voltage waveform is a broad bimodal (double peaked) curve. To increase etching selectivity, it would be desirable to narrow the IEDF. The U.S. patent to Otsubo, U.S. Pat. No. 4,622,094, describes the use of a shaped bias voltage directly coupled to the electrode on which the workpiece or substrate is mounted to reduce the IEDF to a single peak rather than a double peak. However, it would be desirable to further reduce the width of the IEDF function to enhance selectivity and to be able to selectively control the ion peak energy and the ion current. Further, for the etching of high aspect ratio trenches (the ratio of depth to width of the trench) the differential charging effect becomes significant and modifies the IEDF at the bottom of the trench from that at the workpiece surface.
Differential charging occurs in the etching of dielectric materials or materials with a dielectric sub-layer. The electrons in the plasma have a much higher temperature than the ions and arrive at the surface of the substrate with a nearly isotropic distribution. A large fraction of the electrons thus will strike the top or upper portions of the sidewalls of the deep trench in a dielectric material (such as a photoresist), charging these surfaces negatively. The ions are anisotropic and arrive at the surface almost completely normal to the surface as a result of the potential drop across the sheath. The ions move directly down the trench and collect at the bottoms of the trenches, which charge positively. The negatively charged trench sidewalls may even further limit electrons from reaching the trench bottom. As a consequence, the trench bottom is charged to a potential such that relatively lower energy ions will be deflected and may strike and etch the sidewalls, consequently inducing profile defects. Even the higher energy ions are decelerated by the positive potential at the trench bottom, and consequently the etching rate at the trench bottom is lowered, and can even be stopped.
For a broad bimodal IEDF, which is induced by a sinusoidal bias voltage waveform, the potential at the trench bottom is located between the two peaks in the IEDF. For an IEDF having a single peak, the trench bottom potential will be within the relatively narrow range of the ion energy peak. If two plasma processes are carried out with the same average ion energy, but one with a broad IEDF and the other with a narrow IEDF, the deflected ions for the narrow IEDF process will carry higher energy than the deflected ions in the broad IEDF process, and, consequently, the ions striking the trench bottom surface for the narrow IEDF process carry lower energy than the ions striking the trench bottom in a broad IEDF process. As a result, a single peaked IEDF may induce more severe profile defects and ultimately a slower etching rate for deep etching. In addition, in some cases a narrow peak IEDF may induce higher trench bottom potentials which will increase the tunneling current flowing through a thin gate oxide, potentially damaging the oxide.